Donald Tusk expressed his concern about the political volatility in London as he called an extraordinary summit to “finalise and formalise” the Brexit deal that has been struck with the British prime minister.

In a clear sign of the nervousness felt in Brussels, the European council president said leaders would gather on the morning of Sunday 25 November unless “something extraordinary happens”.

Standing next to Michel Barnier, the EU’s chief negotiator, Tusk laid out his plans to consult with the member states, though he urged them not to act on their concerns about the UK being given a customs union.

Tusk said: “I took good note of prime minister May’s statement yesterday. Of course I don’t share the prime minister’s enthusiasm about Brexit as such. Since the very beginning we have had no doubt that Brexit is a lose-lose situation and our negotiations are only about damage control.”

He called for EU ambassadors not to make “too many comments” when they scrutinise the legal text on Friday, before a meeting of ministers from the 27 member states on Monday. But it appeared to be the situation in the UK that was of most concern to the former Polish prime minister.

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“If nothing extraordinary happens, we will hold a European council meeting to finalise and formalise the Brexit agreement. It will take place on Sunday 25 November at 9.30am,” he said. “Finally, let me say this to our British friends: as much as I am sad to see you leave I will do everything to make this farewell the least painful possible, both for you and for us.”

Tusk said the EU ambassadors would also look at revising the mandate given to Barnier to allow him to flesh out the seven-page draft political declaration on the future relationship already published.

Moments earlier, Barnier standing next to Tusk sought to sell the deal as “fair and balanced”, although again he raised his concerns by insisting that the “work is not finished we still have a long road ahead of us on both sides”.

On the completion of loose ends in the deal before ratification of it by Westminster and European parliament, he said: “We have no time to lose”.

Theresa May is likely to have a torrid time when she makes a Commons statement with Brexiters considering a leadership contest. There is also growing anticipation that there will be at least one cabinet resignation.

The Conservative MP Nadine Dorries has claimed that Graham Brady, who chairs the all-important 1922 committee of backbench Tories, has received the 48 letters from MPs required to trigger a vote of no confidence in the prime minister by lunchtime on Thursday.

“I wouldn’t be surprised if they were already in,” she said on Wednesday evening. “There’s going to be more than 48, I’m sure of that, just by the number of people I’ve spoken to tonight. I think we will have the vote of confidence.”

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The prime minister’s former chief of staff, Nick Timothy, has also attacked his former boss in an article in the Daily Telegraph. “It is a capitulation not only to Brussels, but to the fears of the British negotiators themselves, who … never believed Brexit can be a success”, he writes. “This includes, I say with the heaviest of hearts, the prime minister.”

The EU’s member states have in turn expressed their concerns about the deal, raising the spectre of leaders toughening up the terms at the November summit. France, Spain and Denmark raised concerns with the European commission at a meeting on Wednesday evening that the UK was being handed a customs union without sufficiently strong conditions to ensure the UK could not economically undercut Europe.

The Netherlands was also understood to be among those with deep concerns that the British government was being handed too much in the last hectic weeks of the negotiations without sufficient guarantees.

The member states are likely to make their point at a meeting of EU ministers on Monday that the level playing field commitments on tax, the environment and social and labour regulations need to be tougher.

Member states have also flagged up that the commission had granted the UK a customs union offering tariff-free trade without getting assurances on the future fisheries deal.

After a five-hour cabinet meeting on Wednesday night, May had emerged from Downing Street saying there had been a “collective cabinet decision” to agree to the withdrawal agreement and draft political declaration.

It has emerged that nine of the cabinet had argued against the deal. The Brexit secretary, Dominic Raab, reportedly told his colleagues it was an “indefensible” agreement while the work and pensions secretary, Esther McVey, “aggressively” sought a vote and had to be talked down by the cabinet secretary, Mark Sedwill.

May, however, urged her MPs to look beyond the 585-page text when considering their next move. A meaningful vote on the deal in the Commons is likely to be held on 18 December.

On the steps of Downing Street, the prime minister said “decisive progress” had been made. “When you strip away the detail the choice before us is clear: this deal which delivers on the vote of the referendum, which brings back control of our money, laws and borders, ends free movement, protects jobs, security and our union; or leave with no deal or no Brexit at all,” she said.

Under the withdrawal agreement, the UK will enter into an indefinite customs union with the EU if no solution is found to prevent a hard border in Northern Ireland by the end of the transition period in December 2020.

The UK and EU have the right to extend the transition period one time by mutual consent, but the end date has been left at “20XX” in the legal text and extra budget contributions will be demanded.

There is no unilateral right of withdrawal from the backstop customs union and the UK will not be able to implement free trade deals during this time. The British government can request to pull out, but it will be discussed by a joint committee.

Only if the EU is seen to be not talking with “best endeavour” to resolve a dispute can it go to an independent arbitration panel with members nominated by both sides, but that will not be a binding decision on Brussels.

The UK has accepted that it would have to stay in “dynamic alignment” with the EU on state aid rules, preventing anti-competitive subsidies for British businesses.

The UK would have put into British law three EU directives on tax: exchange of tax information, country-by-country reporting on investment firms and the EU’s taxation code of conduct

Philippe Lamberts, the leader of the Greens in the European parliament, and a member of the chamber’s Brexit steering group, called for a second referendum to settle the issue in the UK. He said: “The only way to take back control in this world is by working together: a truly Global Britain is one that takes it full place at the heart of the European Union.

“Now that we have the deal on the table, British citizens know what Brexit will really mean for them. That’s why it would be fair to ask them whether Brexit is the future they want. If not, we would warmly welcome their continued EU membership.”